Wettability control in an oil recovery process

ABSTRACT

For a reservoir rock having constant lithology (i.e. a given rock type and/or grain size) there exists a linear relationship between the log-log plot of sigma (interfacial tension) cosine theta (contact angle) vs. phi (porosity) Swi (irreducible water saturation). The contact angle (measure of the wettability of the reservoir rock) can be determined at in situ conditions from the above relationship, phi and Swi obtained from logging data, and sigma representative of the reservoir fluids. With this in situ determined theta , a more efficient oil recovery process can be designed. For example, if the contact angle is less than 90*, the surfactant within the displacing fluid should have an HLB (hydrophil-lipophil balance) within the range of about 7-40 but if the contact angle is above 90*, the HLB number should be about 0-7. Such a design criteria is especially useful with petroleum sulfonates to obtain emulsion and micellar systems.

I United States Patent [15] 3,643,738

Dreher et al. Feb. 22, 1972 [54] WETTABILITY CONTROL IN AN OIL 3,480,080 11/1969 Murphy ..l66/252 RECOVERY PROCESS 3,482,632 12/1969 Holm ..166/275 X [72] lnventors: D. Delller; Michael Holmes, both of Primary Emmi-MP4 Calvert Attorney.loseph c. Herring, Richard c. Willson, Jr. and Jack [73] Assignee: Marathon Oil Company, Findlay, Ohio Hummel [22] Filed: May 28, 1970 [57] ABSTRACT [21] Appl L477 For a reservoir rock having constant lithology (Le. a given rock type and/or grain size) there exists a linear relationship [52] US. Cl ..l66/252, 166/268 between the log-log plot of 0' (interfacialtension) cosine 0 [51] Int. Cl. ..E2lb43/16, E2lb 49/00 (contact angle) vs. (I; (porosity) S (irreducihlc water satural Field Search 1 tion). The contact angle '(measure of the wettability of the 7 152 reservoir rock) can be determined at in situ conditions from the above relationship, 4; and obtained from logging data. [56] Reierences and arepresentative of the reservoir fluids. With this in situ determined 0, a more efficient oil recovery process can be UNITED STATES PATENTS designed. For example, if the contact angle is less than 90, the 3,288,213 11/1966 King etal ..l66/274 surfactant within the displacing fluid should have an HLB 3,254,714 6/1966 garty (hydrophil-lipophil balance) within the range of about 7-40 2,792,894 957 ham t X but if the contact angle is above 90, the HLB number should Berry Ct be about Such a design cn'terla is especially useful Foster X petroleum ulfonates to obtain emulsion and mlcellar ystems 3,468,377 9/1969 Dunlap et a1. ..l66/274 3,476, l 84 1 H1969 Davis ..166/275 X 6 Claims, 2 Drawing Figures O'l lntertaciol Tension) Cos 9 l contact angle l 4) l porosity l0 Svli (irreducible water Saturation PLOT or a cos 9 vs 11 Swi PAIENIEBFEB22 I972 Y 3.643.738

FIG. 2

m l m z: porosity Swi (irreducible water Saturation) PLOT 0F 0' cos 9 vs Swi INVENTOR MICHAEL- HOLMES KARL D. DREHER 350 400 450 500 Equivalent Weigh? HLB vs EQUIVALENT WEIGHT OF AMMONIUM PETROLEUM SULFONATES A TTORNE Y WETTABILITY CONTROL IN AN OIL RECOVERY PROCESS CROSS REFERENCE TO RELATED APPLICATIONS The linear relationship of cosine 0 vs. 8 on a log-log plot is taught in copending patent application, Ser. No. 41,480 filed May 28, 1970, inventors Michael Holmes and Karl D. Dreher.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to the design of secondary and tertiary recovery processes wherein surfactants are used. From logging data, the contact angle is determined, and this contact angle used to more effectively select the surfactant used in the displacing fluid.

2. Description of the Prior Art It has been established in Using Log-Derived Values of Water Saturation and Porosity," R. M. Morris and W. B. Biggs, Annual Symposium of Society of Professional Well Logging Analysts, 1967, Denver, Colorado, that in a reservoir having constant lithology at irreducible water saturations, the product of d) (porosity) S (irreducible water saturation) is a constant.

The contact angle 0," the formation fluid makes with the reservoir rock measured through the water, can be determined by standard laboratory procedure. However, such is subject to error since the core sample, on which the 0 is determined, is generally not representative of the in situ reservoir conditions. More specifically, the methods of preserving the core adversely influence the resulting 0. Also, atmospheric conditions, drilling mud contamination, etc., adversely influence the laboratory derived 0. Thus, contact angles obtained by laboratory procedures are subject to error.

In designing a secondary or tertiary recovery process wherein surfactants are used in the displacing agent, it is known that the HLB (hydrophil-lipophil balance) number determines the characteristic of the surfactant to emulsify and solubilize the oil and water within the reservoir. Also, the particular displacing agent containing the surfactant should be designed to be compatible with the reservoir rock and to reduce the in situ contact angle to a value closer to 0. Such can be effected by incorporating the correct surfactant within the displacing agent.

SUMMARY OF THE INVENTION Applicants have discovered a novel method of designing a more efficient flooding process by determining the contact angle or wetting angle of the reservoir fluids at in situ conditions. The contact angle at in situ conditions is obtained by first establishing the relationship of o- (interfacial tension) cosine 0 (contact angle) vs. 4) (porosity) S (irreducible water saturation) on a log-log plot. This can be obtained with core samples using mercury/air and distilled water/air and other systems as defined below. After the relationship is established, the d) and S, for the particular zone to be flooded can be obtained from logging data, the 0- determined in a laboratory and the d) deduced by the linear relationship. By having an accurate 0, the flooding process can be efficiently designed. For example, if the 0 is above 90, the surfactant should have a HLB number of O-7 whereas if the contact angle is below 90, the HLB number should be about 7-40.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 exemplifies the relationship that can be established from a reservoir having constant lithology.

FIG. 2 shows the relationship of HLB vs. the equivalent weight of ammonium petroleum sulfonates.

DESCRIPTION OF THE PREFERRED EMBODIMENTS On the ordinate, the product of a cosine 0 is plotted vs. the product of d) S, on the abscissa. Plotted on the log-log graph,

this relationship is linear. This 4) S relationship is established by obtaining core samples from the reservoir, testing the samples in a centrifuge system or positive pressure system to obtain the plot of capillary pressure vs. saturation of the core. Systems, such as distilled water/air, toluene/distilled water, and tetradecane/distilled water, the air and distilled water being displaced from the core are useful. A mercury/air system is also useful to obtain data; a clean core saturated with air is subjected to displacement by mercury under a positive pressure and the saturation of the mercury vs. the capillary pressure is plotted. For the distilled water/air system, the core is saturated with air and then subjected to centrifugal forces so that the capillary pressure vs. the water saturation relationship is established. The same holds true for the toluene/distilled water and tetradecane/distilled water systems. From such relationships, the irreducible saturations of the core are determined for the particular system. The contact angles for the mercury/air and distilled water/air systems are and the a (interfacial tension) of the mercury/air system is 487 dynes/centimeter at 15 C. whereas for the distilled water/air system it is 72 dynes/centimeter at 15 C. For the toluene/distilled water, tetradecane/distilled water, and optionally other systems, the 0 and a are calculated from laboratory data. The 4) is determined by standard laboratory techniques and is representative of continuous porosity. The S is determined from the above capillary pressure data. The product of acosine 0 is plotted on a log-log graph vs. the product of 11: S FIG. I represents such a relationship. The 4: and S for a particular portion of the reservoir is obtained from logging data, the oof the reservoir fluids calculated in a laboratory, and 0 determined from the log-log relationship representative of the reservoir. A more efficient flooding process can be designed with this in-situ determined 6.

Fluid saturations are directly influenced by the contact angle (rock wettability) in an oil-bearing subterranean formation. Such has a pronounced influence on the flow of fluids in the formation. Control of the wettability, or the contact angle, is important in oil recovery processes if an efficient and economical process is to be obtained. Regulating or controlling the contact angle or wettability of the surface can be accomplished by selective absorption and desorption of surface active agents in the displacing fluid used to recover the oil from the subterranean formation. That is, by choosing the correct surfactant, for example in an emulsion or micellar dispersion flooding process or in any flooding process wherein surfactants are used, improved oil recoveries can be realized. Other additives optionally incorporated into the displacing fluid should be chosen such that the overall characteristic of the surfactant is not appreciably changed from the originally designed surfactant.

A design basis for choosing the surfactant is the HLB (hydrophil-lipophil balance) member. That is, surfactants can be characterized as having a certain average HLB number. This HLB number is indicative of the ability of the surfactant to emulsify oil and water. The HLB number indicates the relative distribution of the hydrophilic and lipophilic portions of the surfactant-see Paul Becker's Principles of Emulsion Technology, I955, pp. l04-l09 for definition of HLB. The HLB numbers of different surfactants are algebraically additive; thus a blend of surfactants can have an average HLB number of 7 but can contain HLB numbers of l-40.

The HLB number of a surfactant can be calculated from empirically derived group numbers by the formula (interfacial Phenomena, Phenomena, Davies and Rideal, Academic Press, N.Y., N.Y., 1961, pp. 371-383):

HLB 2(hydrophilic group numbers)m( group number perCH -group)+7 wherein the group numbers are obtained from the following table:

HLB Group Numbers Hydrophilic groups group number SO, Na 38.7 COOK 2L1 COONa l9.l Sulphonate about 1 l Ester (sorbitan ring) 6.8 Ester (free) 2.4 COOH 2.l Hydroxyl (free) l.9

1.3 Hydroxyl (sorbitan ring) 0.5

Lipophilic groups CH p 0475 Derived group (CH CH,-O)- hydrophilic group numbcr=0.33

Petroleum sulfonates have an HLB based on equivalent weight, thus an HLB number of 6 corresponds to an equivalent weight of about 450, an HLB number of 7 for an equivalent of about 420, and an HLB number of 8.0 for an equivalent weight of about 400these equivalent weights are based on ammonium sulfonates. Petroleum sulfonates are preferred as the surfactant within the displacing fluid of this invention. FIG. 2 shows the relationship of HLB vs. equivalent weight of ammonium petroleum sulfonates.

If the contact angles is above 90, the reservoir is oil-wet; and, if the contact angle is below 90, the reservoir is waterwet. This determination is important if it is desired to make an oil-wet reservoir rock water-wet to improve secondary and tertiary recovery methods. For example, if the above is desired, sulfonates having an HLB of about (about 470 equivalent weight) and the proper molecular structure can be blended into a surfactant mix to prepare the slug. Such sulfonates, or their equivalent surfactants, will selectively adsorb onto the oil-wet reservoir rock by means of hydrophobic interaction. The extent of such hydrophobic interactions can be adjusted by choosing sulfonates with the proper aromatic nucleus-cg, the cohesive forces between alkyl naphthalene sulfonates is much greater than between alkyl benzene sulfonates. Therefore, alkyl naphthalene sulfonates are more prone to adsorb and form tightly packed adsorbed layers on the reservoir rock; and, the tighter the packing in the adsorbed layer, the more complete the conversion from oil-wet to a water-wet reservoir rock.

This adjustment of the wettability from completely oil-wet to completely water-wet can be accomplished by blending surfactants having different HLB numbers. If it is desired to make the reservoir rock completely water-wet, sufficient surfactants with the proper HLB number are blended into preferably a micellar dispersion to obtain a wetting angle of 0".

Regarding sulfonates, the polar nature of the sulfonate group causes water to be preferentially adsorbed in a layer coincident with the sulfonate groups. The amount of water adsorbed and the degree of water-wetness imparted to the surface is further controlled by proper selection of counter-ions used in initial preparation of the slug. The hydrophilicy of the sulfonate group depends upon its associated salts and this effect progresses in the order: monovalent divalent trivalent. Within these groupings, e.g., the order is Li Na NH, K and Mg. Ca Ba. Thus, proper consideration of sulfonate structure and salt selection can control the degree to which an oil-wet reservoir is or can be converted to a waterwet system.

If it is desired to convert a water-wet formation to an oil-wet system, the sulfonates used for this purpose should have a relatively high HLB number, i.e., definitely less than the equivalent weight of 470 and preferably less than 420. Also, the emulsion or micellar dispersion should preferably be an oil-ertternal system. I

With a water-wet reservoir rock, that 15, having a contact angle below both oil-external and water-external displacingslugs are useful. Where the formation is flooded with a micellar dispersion followed by a mobility buffer (a slug containing a mobility reducing agent) and this in turn followed by a water drive, it is preferred to leave the reservoir rock waterwet after the micellar dispersion moves through the rock to make it more receptive to the water drive. When a Pusher" polymer (a high molecular weight partially hydrolyzed polyacrylamide marketed by Dow Chemical Company) follows behind the micellar dispersion, the polymer has a tendency to be adsorbed onto the water-wet reservoir rockthe overall effect being to decrease the relative mobility of the mobility buffer; thus imparting a water-wet characteristic to the rock can improve the flooding characteristics by protecting against fingering.

This invention is particularly useful with emulsions and micellar dispersions. But, as mentioned earlier, it is useful with any system containing surfactants. The emulsions and micellar dispersions can contain water, hydrocarbon, surfactant, optionally cosurfaetant (also identified as cosolubilizer and semipolar organic compound) and/or electrolite and other additives if desired, e.g., corrosion inhibiting agents, bacteri cides, etc.

It is not intended that this invention be limited to the particulars taught above. Rather, all equivalents obvious to those skilled in the art are intended to be incorporated within the scope of the invention as defined within the specification and appended claims.

What is claimed is:

1. A method of improving the recovery of oil from a subterranean formation characterized as having substantially content lithology and wherein a flooding agent comprised of surfactant is injected into at least one injection means and displaced through the formation toward at least one production means and oil is recovered through said production means, the method comprising:

1. establishing a log-log relationship of o" (interfacial tension) cosine 0 (contact angle) vs. (porosity) S (irreducible water saturation) representative of the reservorr,

2. obtaining from logging data and S for the particular fonnation within the reservoir,

3. determining a substantially representative of the forma- I tion fluids within the reservoir,

4. from the data of( and (3)" and the relationship of (1, determining and based on this 6 incorporating surfactant having sufficient HLB number and in sufiicient amounts within the flooding agent to substantially reduce the 6 toward a contact angle of 0.

2. The process of claim 1 wherein surfactant having an average HLB number of about 7-40 is incorporated into the flooding agent when the 6 is determined to be less than 90.

3. The process of claim 1 wherein surfactant having an average HLB number of about 0-7 is incorporated into the flooding agent when the 0 is determined to be greater than 90.

4. The process of claim 1 wherein the emulsion.

5. The process of claim 1 wherein the flooding agent is a micellar dispersion.

6. The process of claim 1 wherein the surfactant is a petroleum sulfonate.

flooding agent is i-= 5 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRE a 1,543,738 Dated 2/22/72 lnventofls) Karl D. et. al

It is certified that etror appears in the above-identifiedpatent and that said Letters Patent are hereby corrected as shown below:

Col. 2, line 53: Delete "member" and insert -number.

Col. 4, line 37: Delete "content" and 7 (Claim l) t insert --constant--. Col. 4, line 52: Delete. (1, and

insert (1) Signed and sealed this 6th day of June 1972.

(SEAL) Attest:

'EDWARD M.FLETCHER,-J'R. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents 

2. The process of claim 1 wherein surfactant having an average HLB number of about 7-40 is incorporated into the flooding agent when the theta is determined to be less than 90* .
 2. obtaining from logging data phi and Swi for the particular formation within the reservoir,
 3. determining sigma substantially representative of the formation fluids within the reservoir,
 3. The process of claim 1 wherein surfactant having an average HLB number of about 0-7 is incorporated into the flooding agent when the theta is determined to be greater than 90* .
 4. The process of claim 1 wherein the flooding agent is emulsion.
 4. from the data of ''''(2)'''' and ''''(3)'''' and the relationship of ''''(1,'''' determining theta , and based on this theta incorporating surfactant having sufficient HLB number and in sufficient amounts within the flooding agent to substantially reduce the theta toward a contact angle of 0* .
 5. The process of claim 1 wherein the flooding agent is a micellar dispersion.
 6. The process of claim 1 wherein the surfactant is a petroleum sulfonate. 